During the late 1970's and early 1980's, concern over the emissions from the practice of municipal solid waste (MSW) incineration resulted in the development of more efficient incinerator operating conditions and improved air pollution control technologies. In turn, this enhanced ability to minimise emissions has resulted in the capture of greater volumes of contaminated residues in modern air pollution control (APC) systems. It is generally acknowledged that incineration typically results in a 90% reduction in the overall volume of material for disposal, and a 60 to 75% reduction in weight of municipal solid waste (MSW).
The incineration of municipal wastes is generally recognized to produce the following products: bottom ash and fly ash. Between 70 and 95% of the total ash generated by municipal waste incineration is characterized as bottom ash. Bottom ash typically contains less than 2% combustible material. Due to the extremely high temperatures at which the incineration of municipal solid wastes occur (typically about 1000.degree. C.), there is virtually complete dissociation of organic compounds and volatilization of some metal species, leaving the bottom ash relatively heat stable and chemically inert. Typically bottom ash is relatively insoluble, with only approximately 5% of the total mass thereof being soluble in water.
The remainder of the waste incineration product is classified as fly ash. Fly ash is very soluble in water. For example, up to 30% of heat recovery system ash is soluble in water. The most common species measured in the leachates from fly ash are salts and other flue gas reaction products, specifically chloride and sulphate compounds. Chlorides alone can account for almost 40% of the weight of the soluble fraction of some fly ash. Accordingly, fly ash poses a contamination risk if it is disposed of in such a way that it may come into contact with ground water. Fly ash is considered to be a hazardous waste, necessitating "storage" in underground abandoned mines, or in specialized cells in landfill sites. Alternatively, fly ash must be treated prior to disposal if it is to be disposed of as a non-hazardous material.
Wide spread acceptance and proliferation of municipal solid waste (MSW) or energy-from-waste (EFW) facilities have been tempered by public and government concerns regarding the lack of environmentally acceptable means to dispose of classified residues from the incinerator operations. In most countries around the world, the residues generated within the air pollution control (APC) unit operations of an EFW incinerator are considered classified due to the high concentrations of readily soluble salts and potentially soluble trace metals. As a consequence of these concerns, disposal of the APC residues is one of the major issues limiting the acceptance of new EFW incinerator facilities.
To comply with increasingly stringent air emission regulations, modern air pollution control (APC) systems are designed to cool and chemically condition incinerator flue gases. The hydrogen chloride (HCl) and sulphur dioxide (SO.sub.2) gases that are formed during the combustion of MSW are humidified to reduce the gas temperature to below 150.degree. C. Cooling the incinerator off-gases promotes condensation of vaporous contaminants and enhances certain chemical reactions. This chemical conditioning is generally facilitated by the injection of powdered hydrated lime, or some other form of caustic solution, into the flue gas stream to act as an acid gas sorbent and provide reactive surfaces for condensation of volatile compounds. There are two major types of APC systems: i) wet lime injection and ii) dry or semi-dry lime injection. Acid gas neutralization reactions in dry and semi-dry lime injection APC systems result in the significant production of a predominantly calcium chloride (CaCl.sub.2) and calcium sulphate (CaSO.sub.4) salt waste residue stream. The CaCl.sub.2 and excess lime present in the APC residues are not solubility limited and therefore are released from the solid matrix quickly upon contact with water. In fact, up to 85% of some APC ash residues are water soluble. Furthermore, since the stoichiometric ratio of the lime addition during chemical conditioning is greater than 1, the APC ash residues are highly alkaline, and the potential to solubilise amphoteric metal compounds, such as certain lead compounds, is greatly increased. Consequently, leachates from these residues may contain high concentrations of salts and trace metals including aluminum, lead, chromium and zinc.
Today, disposal of hazardous APC residues is one of the major issues limiting the acceptance and proliferation of new MSW incinerators. It has been argued that the benefits derived from landfill solid waste diversion and energy recovery from MSW incineration are negatived by the precautions which must be taken in the disposal of hazardous APC residues. The risk of leachate contamination of soil and ground water gives rise to long term environmental concerns. In response to these concerns, legislators around the world have generally responded by drafting guidelines that recommend pre-treatment of APC residues prior to disposal. Regulatory guidelines are based on the different characteristics of the two ash streams generated from municipal incinerators. Typically, incinerator design, operation and the type of air pollution control (APC) system each impact upon the residue characteristics. In particular, these factors are responsible for the partitioning of trace metals between the bottom ash and fly ash residues, and the overall solubility of the fly ash residues (see Sawell, S. E. and T. W. Constable. The National Incinerator Testing and Evaluation Program: A Summary of the Characterization and Treatment Studies on Residues from Municipal Waste Incinerators. Environment Canada Report, 1993). The fly ash/APC residue mixture is highly soluble (up to 65% soluble) By contrast, bottom ash is only about 5% soluble. Therefore, bottom ash is generally classified as acceptable for disposal in a sanitary landfill sites, whereas fly ash/APC residue is considered to be a classified hazardous waste due to the high concentrations of readily soluble salts and potentially soluble trace metals. There are considerable costs and significant future liability considerations associated with the generation (and storage/disposal) of classified hazardous wastes, as compared with the storage or disposal of non-classified wastes. In Canada, the Canadian Council of Ministers of the Environment (CCME) have published guidelines that specify that APC system residues should be collected and processed separately from the rest of the incinerator ash streams in order to allow for treatment prior to disposal. (see Operating and Emission Guidelines for Municipal Solid Waste Incinerators Report CCME-TS/WM-TRE003, June 1989.)
After disposal, solid wastes may come into contact with a leachant, such as rainwater, open surface water, or groundwater. Although attempts are made to isolate waste materials to prevent them from coming into contact with water, it is rarely possible to completely prevent contact with water. Accordingly, some of the constituents in the disposed of waste may dissolve into the leachant. Water is continually cycling between the atmosphere, surface water, and groundwater, such that the leaching of solid wastes and the subsequent transport of dissolved waste constituents can have far-reaching environmental implications. The kinds of solid wastes which are of greatest concern for leaching are incinerator, fly and other combustion ashes; sludges and cakes from physical and chemical wastewater treatment operations; contaminated soils; foundry slags; mine tailings; tank bottom sludges, etc. (These wastes are disposed of in the form of dry powders, slurries, and sludges, and may contain a wide range of organic and inorganic constituents. The constituents that are potentially hazardous to the environment are termed contaminants and their presence in potable water should not exceed drinking water quality standards. Once contaminants have been contacted by a leachant, leaching may ensue. Leaching encompasses the physical and chemical reactions that mobilize a contaminant, as well as the mechanisms of transport that carry the contaminant away from the waste. In the classification of materials as hazardous or non-hazardous for disposal purposes, the tendency of the contaminative substances to leach is the predominate consideration. For example, lead is considered to be a contaminative parameter, but if the lead is present in an insoluble compound such as lead oxide, it is highly resistant to leaching in water, and thus not of great concern if present in significant quantities in a waste material. By contrast, lead in the form of a chloride would be readily dissolved in leachates (such as groundwater) such that the presence of significant quantifies of lead chloride would render a waste material hazardous. Thus, leaching tests are conducted according to standardized testing methods to determine the hazardous nature of constituents, and classification of materials is based upon regulatory leachate limits. An example of one such regulation is British Columbia Regulation 132/92 under the Waste Management Act. In this Regulation, leaching test protocols are specified and regulatory leach limits are established. A sample of an ash residue to be placed in a waste disposal area is tested according to the prescribed test protocol, and if the concentrations of contaminative constituents do not exceed the specified regulatory leach limits, then the material is classified as non-hazardous and can be disposed of accordingly. Hazardous materials, so tested and classified, must either be further treated prior to disposal, or disposed of as classified hazardous wastes, and subject to specialized storage protocols.
Globally, several methods have been used in attempts to treat energy-from-waste incinerator APC residues. The most commonly attempted methods are thermal treatment or vitrification; metal recovery; and solidification or stabilisation of hazardous components. Because of the complex chemical matrix of the APC residues, these techniques have generally been costly, and most do not significantly reduce the volume of waste ultimately deposited at landfill sites.
Vitrification generally involves exposing ash to temperatures in excess of 1200.degree. C., with the intent of incorporating the contaminants into a chemically inert matrix. However, vitrification techniques have typically been used on combined ash or bottom ash and not fly ash alone. In addition, the vitrification process is energy intensive and often requires the addition of other materials to retain contaminants in the melt. Consequently, the cost of most full-scale operations is very prohibitive. Finally, and perhaps more importantly, the contaminants of concern in the APC ash residues are volatile and tend to re-vaporize when exposed to the high temperatures of the process. Thus, the off-gas emissions from the vitrification treatment process will require further air pollution control treatment before discharge into the atmosphere, thus negating the original intent of the treatment process.
At the present time, there are some known test-scale systems which recover acids (such as HCl), salts (including NaCl), and gypsum from APC residues; however, these recovery systems are generally limited to wet residues generated from the wet lime injection air pollution control systems. These methods have not proven amenable to the treatment of dry or semi-dry APC system residues.
Ideally, given the concentrations of some metals such as lead and zinc typically present in APC ash residues, significant benefits could be realized from the recovery of metals from these residues. Unfortunately, at the present time, the complex chemistry of the APC residues makes it very difficult to isolate and accumulate many of the trace metals present in the residues in forms pure enough for reprocessing and recycling. One exception is the recovery of mercury. A process commonly known in the art as the German "3R" process is capable of recovering mercury from APC residues; however, this process is limited, in that it can only be used on the residues from wet scrubber system applications. There are currently no known commercial metal recovery systems in operation for the treatment of dry and semi-dry APC ash residues.
Solidification generally implies a process which mixes the APC residues with pozzolanic (cement-like) materials to reduce the leaching of contaminants by reducing the surface area available for contact with water (physical encapsulation). The term solidification also infers the maintenance of a highly buffered environment which limits trace metal solubility (chemical stabilisation). It is a relatively cost-effective process; however, researchers have observed that solidification techniques are not wholly effective. (see Hartlen, J, and A. M. Fallman. "New Perspectives on the Management of Residues from MSW Incineration in Sweden" Proceedings of the 7.sup.th International Recvcling Congress, Waste Management International, Ed K. J. Thome-Kozmiensky, Pub. EF-VERLAG fur Energie-und Umwelttechnik GmbH, Berlin, Germany, 1992.) Since a substantial proportion of APC residues consist of soluble chloride salts, major problems have been encountered with the loss of structural integrity and durability of the solid matrices due to salt leaching. Moreover, the highly alkaline nature of the matrix can lead to the leaching of amphoteric metal compounds, such as lead (Pb) and zinc (Zn) compounds. One known and commercially available stabilization process requires the addition of large volumes of phosphoric acid to the APC residue to convert soluble lead (Pb) to insoluble lead phosphate compounds. Although this process is reasonably effective at reducing lead solubility, the process does not prevent salt leaching. Moreover, this process adds approximately 40% (acid+water) weight to the remaining treated residues, and this increase in volume brings a concurrent increase in the demand for landfill storage space, and associated costs.